G2iii: (Mock) Describe the cardiovascular events that occur during ventricular systole

Systole = the period of the cardiac cycle where blood is ejected from the ventricles

  • Events of systole divided into:
    1. IONIC
    2. MECHANICAL
    3. ECG ∆

Ionic

  • Cardiac AP from SA Node depolarises myocyte

→ Threshold potential -65mV reached

→ Fast Na+ channels open = RAPID DEPOLARISATION

→ TRANSIENT OUTWARD K+ channels opens = INITIAL REPOLARISATION

→ L TYPE Ca2+ CHANNELS open @ –40mV = PLATEAU PHASE

Excitation-Contraction-Coupling

  • This influx Ca2+ causes further Ca2+ release by myocyte SR
  • ↑intracellular Ca2+
  • Ca2+ binds TnC
  • Conformational ∆ in Tropomyosin uncovers Actin Binding Site
  • Allows myosin head to form a cross bridge
  • Sarcomere shortens until ↓Ca2+ or ↓ATP supply
  • Contraction of cardiac myocyte

ECG

  1. ISOVOL CONTRACTION
    • spreads from AV node → Purkinje fibres
    • Ventricular depol. = QRS complex
    • Simultaneously atria are repolarising (atrial T wave not seen on ECG because covered by QRS)
  2. RAPID EJECTION
    • Ventricles completely depolarised at beginning of ejection
    • ISOELECTRIC ST SEGMENT on ECG
  3. REDUCED EJECTION
    • Ventricular repolarisation → T wave

Mechanical Events

There are 3 phases:

  1. ISOVOL CONTRACTION
  2. RAPID EJECTION
  3. REDUCED EJECTION

1) Isovolumetric Contract

  • All valves closed
  • Ventricles depolarise
  • Myocyte contraction causes ↑intraventricular P
  • Ventricular P > atrial P = AV valves shut
  • Rapid ↑ ventricular P without ∆ volume
  • Maximal rate of ↑P KA dP/dt

2) Rapid Ejection

  • Ventricular P > aorta & pulmonary pressures
  • Ejection of blood
  • Maximal systolic aorta & pulmonary pressures reached

3) Reduced Ejection

  • Kinetic E of blood continues to propel blood outward, but at lower pressures